For evaluating the performance of an internal combustion engine and the corresponding engine design development, it is desirable to provide dynamic measurements of engine oil consumption during operation of the internal combustion engine. Early methods for determining oil consumption included the use of a dipstick, drain-weigh techniques, sulfur methods and radiometric techniques. However, each of these methods have serious shortcomings, and all fail to provide real-time analysis of oil consumption. Radiometric methods originally employed in the art provided a very precise method for measuring oil consumption through the addition of a radioactive bromine tracer 1,2-dibromooctadecane to the engine oil. With this method, the resultant combustion product from the internal combustion engine is trapped within a sodium hydroxide solution and counted by scintillation counting. Though extremely accurate, this method is undesirable because of the significant radioactive health and safety considerations and regulatory requirements necessary for its use. Another shortcoming is that radiometric methods are essentially batch processes which do not readily lend themselves to individual measurements, and require the preparation of a fresh radioactive bromine tracer for each batch operation because of the short half life of the tracer.
As a solution to the above, U.S. Pat. Nos. 4,990,780 and 5,445,964 to Lee et al., assigned to the assignee of this invention and incorporated herein by reference, provide a method for determining oil and fuel consumption that is relatively simple and precise, enables real-time measurements. Each of these patents teaches the use of nonradioactive tracer compounds, such as bromine in the form of organic bromo-compounds, which are added in small amounts to the engine oil. When combusted in the presence of hydrocarbon compounds in oil and fuel, the bromine constituent of the particular compound is converted into hydrogen bromide (HBr). A sample of the exhaust gases generated by the internal combustion engine and containing HBr is then analyzed within a sample cell, where the gas sample is maintained at a pressure at which a distinction between an absorption line of the tracer specie and the absorption lines of a related isotopic species can be discerned. Monochromatic radiation is then transmitted through the gas sample at the frequency of an absorption line for the tracer specie. Tunable diode laser spectroscopy is preferably used with collimating and collecting optics that are preferably reflective, off-axis parabolic or focusing mirrors. The sample cell is a long-path multi-pass all-reflection absorption cell or a waveguide sample cell to achieve the high sensitivity required for analysis of the extremely minute gas component.
In use, the engine exhaust measurement systems taught by Lee et al. have performed extremely well. However, under some engine operating conditions, the tracer compound has been found to incompletely combust, with the result that HBr is not completely formed and the spectrometer is unable to detect the entire specie in the engine exhaust. Also under some operating conditions, partially burnt products of combustion from the engine exhaust have been found to adhere to the mirrors of the sample cell and render them unusable after a short period of time.
Therefore, it would be desirable if improvements in the teachings of Lee et al. could be achieved by which the exhaust gas of an engine was fully combusted to ensure that gaseous HBr is present and detectable, without undesirably altering the gas sample or the performance of the engine exhaust measurement system.